High energy neutral emissions from solar flares

Toner, Mark P (2004) High energy neutral emissions from solar flares. PhD thesis, University of Glasgow.

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Solar flares produce two kinds of high-energy neutral emissions: gamma-rays and neutrons (with thanks to Chupp 1984, for this convenient title). These originate in interactions between accelerated ions and ambient nuclei and, between the photons and the neutrons, provide a window into the fast ion population between the energies of a few MeV and a few hundred MeV. Previously, the gamma-ray spectrum has been investigated by Monte Carlo modelling methods and conditions in the flare region deduced by adjusting the models to fit the observed spectra, chiefly from the Solar Maximum Mission Gamma Ray Spectrometer (e.g. Murphy et al. 1991). This provides abundances for accelerated ions and ambient nuclei, and energy distributions for the accelerated ions. However a number of assumptions have to be made in order to keep the model parameter numbers low enough to enable statistical matching to the data in a reasonable amount of processor time and there are some difficulties with the parameters deduced. In particular, there are difficulties in accounting for the brightness of the narrow emission line at 1.63 MeV due to the deexcitation of 20Ne. This demands a steep accelerated ion spectrum and a low abundance for fast alpha-particles. However, strong emission in the 0.429 MeV and 0.478 MeV features due to the production of lithium and beryllium by fusion of alpha-particles often contradicts this, demanding an enhanced population of accelerated alpha-particles. Fresh approaches to the modelling of some of the narrow lines are presented here in order to explore, and possibly resolve, this contradiction. Two possibilities are examined; the assumptions in previous work of a cold target region and the further assumption that all of the accelerated ions share the same energy distribution. It may be that the accelerated ions pass through a region in which, at least for the lower energy ions, energy loss is experienced in warm target conditions (e.g. Emslie et al. 1997). If they spend a sufficient part of their time in such a region, their energy distribution will be altered to enhance the number of ions at energies of a few MeV. This would preferentially stimulate the 1.63 MeV line due to the particularly low energy sensitivity of 20Ne to proton-excitation, without having to invoke a steep ion spectrum. Previous attempts to apply this analysis did not model the line production in detail and considered only the peak values of the excitation cross-sections. Integrating the excitation in detail allows a resonant spike at 3 MeV in the 20Ne cross-section to have its effect and reduce the required proton energy slope. Even with a large proportion of accelerated alpha-particles, this approach can reduce the ion slope and significantly reduce the required ion energy in the flare. The second assumption challenged here, is that all of the accelerated ions have the same energy distribution, characterized by a single power-law index. Relaxing this assumption and modelling the lines with separate fast proton and fast alpha-particle power-laws has significant results. It is possible to satisfy the bright 1.63 MeV line and the strong emission from the alpha-fusion reactions with a steep proton spectrum and a hard alpha-particle spectrum. This also significantly reduces the required alpha-particle abundance to values more in line with cosmic abundances. Rounding out this work on high-energy neutral emissions, the GUIPS software has been applied to correctly inverting a Compton Gamma Ray Observatory COMPTEL neutron spectrum from the solar flare of 15 June 1991. Maximum entropy and quadratic inversion techniques are applied to the instrument data to produce an improved neutron spectrum. Previous simplified approaches have underestimated the sensitivity of the instrument as well as distorting the neutron spectrum. A dip in the spectrum has been confirmed but the overall number of neutrons detected has to be reduced to 73% of the previous value obtained by Kocharov et al. (1998). Further investigation of COMPTEL neutron spectra awaits the preparation of the data. This depends on extensive work modelling the passage of the solar neutrons through the instrument using the Los Alamos LAHET and MCNP software.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Astronomy
Date of Award: 2004
Depositing User: Enlighten Team
Unique ID: glathesis:2004-74203
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 23 Sep 2019 15:33
Last Modified: 23 Sep 2019 15:33
URI: http://theses.gla.ac.uk/id/eprint/74203

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